This invention relates to a piezoelectric actuator having a piezoelectric element as a drive source on a vibration plate surface, a liquid jetting head incorporating such an actuator, and a method of manufacturing method such an actuator and such a head.
Piezoelectric elements become deformed as electric energy is supplied, and are used widely as drive elements for a liquid jetting head, a micropump, and a sounding body (loudspeaker, etc.), for example. Here, the piezoelectric element comprised of a piezoelectric material which shows a piezoelectric effect, such as a piezoelectric ceramics made by compressing and baking metal oxide powder (e.g., BaTiO3, PbZrO3, PbTiO3), a piezoelectric polymer film using polymer compound. The liquid jetting head ejects a liquid droplet from a nozzle orifice by causing pressure fluctuation to occur in liquid in a pressure chamber; the liquid jetting heads include a recording head used with an image recording apparatus such as a printer, a liquid crystal jetting head used for manufacturing a liquid crystal display, a color material jetting head used for manufacturing a color filter, and the like, for example. The micropump is a microminiature pump capable of handing a trace amount of liquid; for example, it is used to send an extremely small amount of a liquid medicine.
One of important parts used with such a liquid jetting head and a micropump is a piezoelectric actuator having a piezoelectric element on the surface of a vibration plate. The piezoelectric actuator is attached to a chamber formation plate having a hollow to be a pressure chamber which is partly defined by the vibration plate. To eject a liquid droplet or send liquid, a drive pulse is supplied to the piezoelectric element for deforming the piezoelectric element and the vibration plate (namely, the deforming portion of the pressure chamber) and thereby changing the volume of the pressure chamber.
In the liquid jetting head and the micropump, there is strong demand for high-frequency driving of the piezoelectric element to realize high-frequency ejecting of liquid droplets and enhance the liquid sending capability. To realize high-frequency driving of the piezoelectric element, it is necessary to make compliance of the deformed portion smaller than ever and make the deformation amount of the piezoelectric element larger than ever, because if the compliance of the deformed portion is made smaller, responsibility is improved and thus it is made possible to drive at a higher frequency than ever, while if the deformation amount of the piezoelectric element is made larger, the volume change amount of the pressure chamber grows and thus the amount of ejected liquid droplet and the amount of sent liquid can be increased.
A piezoelectric element having a multilayer structure is proposed to meet the contradictory requirements for the compliance of the deformed portion and the deformation amount of the piezoelectric element. For example, Japanese Patent Publication No. 2-289352A discloses a piezoelectric element having a structure wherein a piezoelectric body layer is made of a two-layer structure of an upper piezoelectric body and a lower piezoelectric body, a drive electrode (discrete electrode) is formed on the boundary between the upper piezoelectric body and the lower piezoelectric body, and the upper piezoelectric body and the lower piezoelectric body are formed each on an external surface with a common electrode. Likewise, Japanese Patent Publication No. 10-34924A also discloses a piezoelectric element having a multilayer structure.
In the multi-layered piezoelectric elements described above, the drive electrode is placed on the boundary between the upper piezoelectric body and the lower piezoelectric body and thus each of the upper and lower piezoelectric bodies is given an electric field of the strength determined by the spacing from the drive electrode to each common electrode (namely, the thickness of each piezoelectric body) and the potential difference between the drive electrode and each common electrode. Thus, as compared with a piezoelectric element having a single-layer piezoelectric body sandwiched between a common electrode and a drive electrode, the double-layered piezoelectric element can be largely deformed as the same drive voltage as ever is applied even if the whole piezoelectric element is slight thickened to enhance rigidity.
In fact, however, the piezoelectric element of the multilayer structure described above is not yet commercially available. Thus, the piezoelectric element having a single-layer piezoelectric body sandwiched between a common electrode and a drive electrode must be used as an actual product. Various reasons are possible; it is also considered as one reason that it is difficult to obtain an assembly including the multi-layered piezoelectric elements having uniform piezoelectric body characteristics.
It is therefore an object of the invention to obtain a piezoelectric actuator provided with multi-layered piezoelectric elements having highly unified piezoelectric body characteristics, and a liquid jetting head incorporating such an actuator.
It is also an object of the invention to provide a method of manufacturing such a piezoelectric actuator and such a liquid jetting head.
In order to achieve the above objects, according to the invention, there is provided a piezoelectric actuator, comprising:
a vibration plate;
a first common electrode, formed on the vibration plate and to be fixed at a predetermined potential, the first common electrode having a first proximal portion and first branch portions extended from the proximal portion;
a first piezoelectric layer, laminated on each of the first branch portions;
a drive electrode, laminated on each first piezoelectric layer;
a second piezoelectric layer, laminated on each drive electrode;
a second common electrode, having a second proximal portion and second branch portions extended from the proximal portion such that each of the second branch portions is laminated on each second piezoelectric layer and to be fixed at the predetermined potential; and
a conduction member, which electrically connects the first proximal portion and the second proximal portion.
According to the invention, there is also provided a liquid jetting head, comprising the above actuator unit such that each of the first branch portions is associated with a pressure chamber communicated with a nozzle orifice from which a liquid droplet is ejected.
According to the invention, there is also provided a method of manufacturing a piezoelectric actuator, comprising steps of:
providing a vibration plate;
forming a first common electrode on the vibration plate so as to have a first proximal portion and first branch portions extended from the proximal portion;
laminating a first piezoelectric layer on each of the first branch portions;
laminating a drive electrode on each first piezoelectric layer;
laminating a second piezoelectric layer on each drive electrode;
forming a second common electrode having a second proximal portion and second branch portions extended from the proximal portion such that each of the second branch portions is laminated on each second piezoelectric layer, while the first common electrode and the second common electrode are insulated from each other; and
connecting electrically the first proximal portion and the second proximal portion.
Preferably, the manufacturing method further comprising steps of:
measuring first piezoelectric characteristics of the first piezoelectric layer before the first proximal portion and the second proximal portion are electrically connected; and
classifying the actuator into one of predetermined ranks in accordance with the measured first piezoelectric characteristics.
Preferably, the manufacturing method further comprising steps of:
measuring second piezoelectric characteristics of the second piezoelectric layer before the first proximal portion and the second proximal portion are electrically connected; and
classifying the actuator into one of predetermined ranks in accordance with the measured second piezoelectric characteristics.
According to the invention, there is also provided a method of manufacturing a liquid jetting head, comprising steps of:
providing the actuator unit manufactured by the above method;
providing a flow passage unit formed with pressure chambers each communicated with a nozzle orifice from which a liquid droplet is ejected; and
joining the actuator unit and the flow passage unit such that each of the first branch portions is associated with one of the pressure chambers.
In the above configuration, since the first common electrode and the second common electrode are electrically insulated when they are formed. the piezoelectric body characteristics can be separately measured for each of the upper piezoelectric body and the lower piezoelectric body. Accordingly, a product having highly unified piezoelectric body characteristics of the piezoelectric elements can be obtained.